Timing information, in terms of frequency, phase (e.g., relative time), and absolute time of day, can be passed over an Ethernet or Internet Protocol (IP) network through the use of a variety of different protocols. Two of the most common such protocols are the Network Time Protocol (NTP) and the Institute of Electrical and Electronics Engineers (IEEE) 1588 protocol, referred to as the Precision Time Protocol (PTP).
NTP is a well established protocol, while PTP is a relatively new standard for communicating timing information. However, both NTP and PTP are packet based protocols that rely on round trip measurements and, as such, are generally referred to as Two-Way Time Transfer (TWTT) mechanisms. The accuracy of the round trip measurements is impacted by Path Delay Variation (PDV) and asymmetry.
DOCSIS is a telecommunications standard that permits the addition of high-speed data transfer to an existing cable TV (CATV) system. It is employed by many cable television operators to provide Internet access (i.e., cable Internet) over their existing hybrid fiber-coaxial (HFC) infrastructure. As a transport, the DOCSIS upstream may be unpredictable and may contain both packet delay variation (PDV) and asymmetry. This causes NTP, PTP, or any external timing protocol to lose significant accuracy when passed over or through a DOCSIS system/network.